A “violent and volatile prison population” is being fuelled by better access to so-called legal highs, a union has said.
Ambulance callouts to Welsh prisoners who used psychoactive substances have doubled in four years.
The Prison Officers’ Association’s Glyn Travis said drugs and staff shortages were creating “turmoil”.
The Ministry of Justice said it was committed to making prisons “places of safety and reform”.
Figures were obtained from Cardiff, Swansea, Parc in Bridgend and Usk prisons by BBC Radio Cymru’s Post Cyntaf programme.
They showed psychoactive substance-related ambulance callouts doubled from 202 in 2012-13 to 406 in 2015-16.
Between 1 April – 31 December last year, there were 281 callouts.
Mr Travis described a type called spice leading to more cases of self-harm, suicide and prisoners being found unresponsive in their cells.
“A prison population is like any community, you’ve got young, old… you’ve got people who are dependent on medication, who are dependent on drugs,” he said.
“But what we’re seeing is that we’re getting a more violent and volatile prison population.
“We believe that one of the underlying causes of that is the accessibility of what people call legal highs.”
Mr Travis said the situation was being exacerbated by a reduction in the number of front line prison staff.
“I would say that prisons are in absolute turmoil – like the rush hour of the tube stations in London on a daily basis.
“You have prisoners milling around with very few staff to supervise them and disorder and ill-discipline can very quickly break out and come to a problem.”
Paramedics told BBC Wales that incidents were also stretching their resources – with a callout taking far longer than one to somebody’s home.
The Welsh Ambulance Service’s Richard Lee said: “When we respond to patients in a prison there are security requirements which we comply with in order to enter or leave the scene which obviously take longer than entering a residential address.”
A Ministry of Justice spokesman said: “We have taken immediate action to stabilise the estate and help tackle the drugs, drones and phones that undermine security.
“We are also investing £100m annually to boost the frontline by 2,500 officers.
“These are long-standing issues that will not be resolved in weeks or months but our wholesale reforms will lay the groundwork to transform our prisons, reduce reoffending and make our communities safer.”